Vacuum Interventions in Radioactive Environments J.M. Jimenez AT Department / Vacuum Group.

Vacuum Interventions in Radioactive Environments

J.M. Jimenez

AT Department / Vacuum Group

AT Vacuum Group: A CERN Wide Mandate by J.M. JimenezOLAV 1 Workshop CERN, April 11-12 2005

Main topics

• Annual Dose Limits• Source of radiations• Typical mechanical interventions• Review of the major problems• How to handle such difficulties?• Implications for future designs


Annual Dose Limits (1)

• Legal Limits:– Dose received during any consecutive 12-month period < 20 mSv. Special restrictions for women of childbearing age

– CERN limit for Contractor’s personnel set at 1 mSv per week, averaged over the time spent at CERN

• Design Limits for the LHC:– Component design shall be compatible with maintenance/operations

with an annual dose limit of 5 mSv.

• Derived Constraints– Optimisation principle of all work in radiation areas is mandatory

(ALARA)– If estimate exceeds 100µSv an optimisation must be made balancing the

doses against the cost of protection measures (time, shielding, distance and remote handling)


Annual Dose Limits (2)

• Long term experience at high-energy accelerators gives the following dose rate reference values:– 100 µSv/h: Above this value all work must be planned and optimised.– 2 mSv/h: Intervention time in the zone must be severely limited. Contractor’s

personnel who only have a temporary contract are not allowed to work in these zones. Remote handling should be seriously envisaged.

– 20 mSv/h: No work is allowed since dose limits would be too easily exceeded. Remote handling is essential.

First limit comes from the definition of Limited-stay area where work in the area requires the authorisation of the Radiation Safety Officer (RSO) and control of dose accumulation by a Radiation Protection Technician together with the wearing of additional dosimeters.The second limit comes from another area classification change, to that of a High-radiation area, where special precautions are needed to prevent the excessive accumulation of dose.Above the third limit, annual design dose of 5 mSv / person would be received in less than 15 minutes, and so real maintenance work involving human intervention is impracticable.


Sources of radiations

• Important:In the old LEP and in the SPS accelerators, contamination

problems were (are not an issue)It could be different in the LHC…

• Sources of radiations:– Direct activation by the nuclear cascade resulting from

beam losses (high energy protons, ions and leptons)– Indirect activation by the nuclear cascade i.e. high energy

photons and secondary neutrons, resulting from the:• synchrotron radiation (high energy photons),• electrons accelerated by RF fields in cavities and lost in their

extremities


Typical Mechanical Interventions

• During the operation of the accelerator– Failures of essential equipments Beam control/dump

– Vacuum leaks:• Fatigue: vacuum chambers in pulsed magnets, welded bellows on

beam monitors

• Corrosion: Hydrochloric and nitric acid, ozone

• Beam losses: protons and ions

• Overheating by synchrotron radiations

• During shutdowns– Repairs of existing equipments

– Installation of new equipments


Review of the major problems (1)

• In the LEP– Damages to cables Accentuated during the mechanical

interventions by the cable displacements– Slight activation of stainless steel transitions (< 2 mSv/h)– Corrosion by PVC

Scotch used to fix the radiation probes onto the bellowsPVC covers on bellows

• In the SPS, same as for LEP plus:– PVC corrosion induced by the use of inappropriate cables

and magnet covers– Leaks on water/vacuum junctions– Holes drilled by the proton beams


2003: A “hot” year for the Vacuum System of the SPS North Area

More than 25 interventions of the Vacuum Piquet in 6 weeks (>4/Week!) 27 peoples involved ~70% of the personnel involved outside the Section in charge of the SPS operation.

Review of the major problems (2)


How to handle such difficulties?

• Slight activation of stainless steel transitions (< 2 mSv/h)– Identify the hot spot(s) and stay as far as possible Dose decreases by 1/d or 1/d2 in the case of a radioactive blocs or of spots

respectively.

• Holes drilled by the proton beams and Leaks on water/vacuum junctions Costly in terms of dose for the leak detections Change the component

• Damages to cables Take the maximum precautions during interventions

• Corrosion by PVC Change all the components attacked by new protected one Clean the components Dry the air to avoid water condensation Remove the source of chloride

• Proceed to predictions and optimisation of doses prior to interventions


Crash program 13 chambers and 14 bellows manufactured and exchanged in only 5 days ! Dose estimations respected, 21 mSv achieved for all interventions

• Area secured by adding Aluminum protection covers…

Preventive exchange of all chambers


Prediction of dosesSPS North Area - Splitters (TDC2)


Cumulated dose after 2 months

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Implications for future designs (1)

• WELDS AND BRAZING:– Number and length of the welds shall be minimized to avoid corrosion– Crossing welds shall be avoided completely– brazing at atmospheric pressure under vacuum are not allowed– brazing fluxes are strictly forbidden for reasons of corrosion

• COOLING CIRCUITS:– Brazing between vacuum and the cooling circuits are not allowed– Fully penetrating welds are not allowedRequired:– Independent cooling circuits in order to be emptied during the bake out

By experience, water (more generally coolant fluids) leaks in a vacuum system are very difficult to locate since they are characterized by a continuous opening and closing of the leak leading to long cuts in the operation of the accelerator and an excessive exposure of the intervention crew radiation.



• LEAK DETECTIONS ON HOT COMPONENTS– Design shall make easier and quicker the in-situ leak detections– Minimum distance between two potential leaking components i.e.

bellows, flanges, feed-through, welds (10 mm minimum) to allow the installation of a plastic bag for local leak detections.

– Proper access to the equipment: cabling, radiation shielding and bake out equipment in particular if permanent.

• BELLOWS– Bellows are the most sensitive components to corrosions due to their

thin thickness (< 0.2 mm)– Hydroformed bellows shall be preferred to welded bellows

• FLANGES– As an alternative to the Conflat® flanges, use of the quick release

conical flange couplings with Conflat® knife design but with chain clamps.



• IMAGINE ALTERNATIVE SOLUTIONS: CASE OF THE SPLITTERS IN THE SPSThe problems of reliability are located in the two splitter areas due to corrosion by PVC. Vacuum chambers, vacuum ports and collimator exchanged but the splitters remain in place: few spares, difficult to install, highly radioactive and already with a lot of traces of corrosion.

New leaks in splitter can not be excluded Solution proposed

– Isolate the corroded splitter area using 0.1 mm mobile Ti windows. In case of leak, the leaking splitter sector will be kept below 10-1 mbar

Existing layoutBSG Collimator

BSG

T2

T4

Proposed layoutBSG Collimator

BSG

T2

T4Sector valve "Movable" or fixed Ti (0.1 mm) window

Splitters

Splitters


Conclusions

• Operation and maintenance induced radiation shall be addressed at the early design stage

• Quantify the risk tochoose the appropriatetechnical solutionregarding future doseimplications.

Remote handlingrequired

Dos

e ra

tes

Reliability

Safe operation

Designoptimisation

Training andinterventionprocedures

Beam stoppers/protection

blocs…Collimators, scrapper…

Wire scanners, beam monitors…

RF cavities, vacuum instrumentation, ion

pumps…

Vacuum Interventions in Radioactive Environments J.M. Jimenez AT Department / Vacuum Group.

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